WebP support was added briefly in the macOS Sierra (10.12) public betas, but was removed. It doesn't seem to work in Safari right now (macOS Sierra 10.12.2 beta), according to this test page:
WebP on the other hand is a proprietary format developed by a single company, an implementation of which is made available under an Open Source License.
So yes it's patent-encumbered but no more patent-encumbered than WebP.
https://chromium.googlesource.com/webm/libwebp/+/master/PATE...
On the positive side, it's now old enough that some key patents (e.g. arithmetic coding) have actually expired.